1. Field of the Invention
This invention relates to electronic engine control of internal combustion engine operation.
2. Prior Art
Exhaust gas oxygen sensors have been used for many years for feedback control of the air-to-fuel ratio. The sensor has two catalytic noble metal electrodes, the outer electrode exposed to the exhaust gas and the inner electrode to the ambient air as a reference.
If P.sub.O,o and P.sub.O,i are the partial pressures of oxygen in the gases in contact with the outer and the inner electrodes, respectively, the sensor develops an emf given by the Nernst equation: EQU emf=RT/4F 1n(P.sub.O,i /P.sub.O,o)
where, R, T, and F are respectively the ideal gas constant, the absolute temperature, and the Faraday constant.
FIG. 2 shows the partial pressure of oxygen P.sub.O in the exhaust gas as a function of air-to-fuel ratio. The solid line corresponds to the case that the exhaust gas is in thermodynamic equilibrium at a given temperature (700.degree. C. in FIG. 2). The main feature is the abrupt and large change of P.sub.O at the stoichiometric A/F ratio. In contrast, the partial pressure of the free oxygen depicted by the dotted lines does not change significantly at stoichiometry and varies depending on the quality of the combustion in the cylinders of the engine. It is apparent that a unique relationship between partial pressure of oxygen and A/F exists only for the case of thermodynamic equilibrium.
It is important that the outer electrode of the sensor is sufficiently catalytic as to bring the exhaust gas close to the thermodynamic equilibrium at the sensor surface. This is the reason for using catalytic noble metals such as platinum and palladium as materials for the electrodes of the sensors. FIG. 3 shows typical HEGO sensor output as a function of A/F for noble metal catalytic electrodes.
Although HEGO sensors have been used successfully for years, several problems have occasionally arisen. One of these problems is the loss of the air reference atmosphere as a result of rich exhaust gas leaking into the air reference compartment through the exhaust gas seal. Two of the main causes responsible for this loss of air reference are the replacement of the metal seal with an electrically insulating ceramic seal which cannot be entirely impervious to gaseous species and the need to make the sensor water-submersible which severely limits the amount of air that can enter the air compartment from the ambient atmosphere. As a result, the oxygen concentration in the reference compartment can be temporarily depleted when the exhaust gas leak becomes sufficiently high because the catalytic inner electrode equilibrates the combustible species leaking from the exhaust side with the oxygen of the reference compartment. Under these conditions, for rich air-to-fuel mixtures, P.sub.O,i and P.sub.O,o are both very small and of the same order of magnitude. Consequently, the sensor emf has a small value. For lean air-to-fuel mixtures, P.sub.O,i is many orders of magnitude smaller than P.sub.O,o and the emf has a large negative value (FIG. 4). This negative shift of the sensor emf vs air-to-fuel ratio is called Characteristics Shift Down.
When the engine is operated in closed-loop control, the air-to-fuel ratio is alternately increased and decreased in time by monitoring when the output of the oxygen sensor crosses the 450 mV setpoint (or similarly calibratable value), which corresponds to stoichiometry in a fully functional device. However, when contamination of the air reference compartment occurs, the sensor emf always remains below the 450 mV value regardless of the air-to-fuel ratio in the exhaust because of the CSD effect. When this happens, the engine controller assumes that the engine is operating leaner than stoichiometry and attempts to compensate for it by shifting the air-to-fuel ratio to the maximum rich allowed value. Emissions are affected under this mode of operation and a MIL indicator is lit to signal the lack of HEGO switching as required by OBD II.
The present invention deals with a method to prevent the air reference from losing its oxygen concentration and prevent malfunctions of the engine control system.